My son who has been diagnosed with PDD has also been diagnosed with adhd. I don't disagree with the diagnoses. Should I seek out second opinion?

Maybe. I guess it just depends how bad it is. The school district may have an evaluative program that is covered by your tax dollars. Start there first. If severe then i would seek a specialist opinion, like neurology, developmental and behavioral, or psychiatry.
Maybe. If treatment is working and you dont disagree with a diagnosis, there would seem to be little practical reason for a second opinion. Pdd has rather loose creterion, and there are some who believe that mild autism sectrum disorder is presently overdiagnosed - many adhd children have social skill problems but not the severeity of pdd, this could be an issue in some future careers.